This file represents a weighted directed graph whose nodes are the numbers
0,1,2,3,4,5,6.  Each line is an edge.  The first and second entries of
a line are the source and target of the edge, and the third entry is the
weight.  For example, the line "2 5 1" means that there is an edge from node
2 to node 5 of weight 1.

Use Dikstra's algorithm to solve the single source shortest path problem,
where the start node is 0.

0 3 2
1 0 1
1 2 6
2 0 2
2 5 1
3 2 2
3 4 4
4 5 1
5 3 5
5 6 1